This invention is directed toward logging of earth formations penetrated by a borehole, and more particularly directed toward the determination of formation gas saturation and other formation parameters from measures of fast neutron and inelastic scatter gamma radiation induced by a pulsed, fast neutron source.
In the context of this disclosure, "logging" is defined as the measure of a parameter of material penetrated by a borehole, as a function of depth within the borehole. Parameters of interest include density, porosity, and the liquid and gas saturation of the formation.
Density logging systems, which are compensated somewhat for the effects of the borehole, were introduced in the mid 1960s in the paper "The Physical Foundation of Formation Density Logging (Gamma-Gamma)", J. Tittman and J. S. Wahl, Geophysics, Vol. 30, p. 284, 1965. The system introduced by Tittman et al, commonly referred to as a compensated gamma-gamma density logging system, was designed to operate in boreholes which are "open" and contain no steel casing. An instrument or "tool" is lowered into the well borehole on a cable, and the depth of the tool is determined by the amount of cable deployed at the surface of the earth. This type of tool contains an intense gamma-ray source and preferably two gamma-ray detectors at differing distances from the source. The gamma ray detectors measure gamma rays which are scattered from electrons in the formation, and back into the borehole. Since for most earth formations, the electron density is in constant proportion to mass bulk density, the "backscatter" gamma ray intensity at the detectors provides a measure of formation bulk density. Two detectors are preferably employed to allow the measurement to be compensated for the effect of mudcake that tends to accumulate on the borehole wall from drilling fluid used in the drilling process.
The gamma-gamma density tool has a characteristic shallow depth of investigation into the formation of about 4 inches ("Depth of Investigation of neutrons and Density Sondes for 35% Porosity Sand", H. Sherman and S. Locke, Proc. 16th Annual SPWLA Symp., Paper T, 1975) and therefore is heavily influenced by the near borehole environment. This tool cannot make quantitative density logs in boreholes which have been cased, where the casing is typically steel and is surrounded by a cement sheath.
One technique for measuring formation porosity utilizes a porosity sensitive tool known in the industry as a "neutron-neutron" porosity system (Dual-Spaced Neutron Logging for Porosity", L. Allen, C. Tittle, W. Mills and R. Caldwell, Geophysics Vol. 32, pp. 60-68, 1967). The downhole tool portion of the system contains a source of fast neutrons which is typically an isotopic source such as Americium-Beryllium (AmBe). Preferably two detectors sensitive to thermal or epithermal neutrons are axially spaced from the source at different distances. The detectors respond primarily to thermal or epithermal neutrons back-scattered into the borehole by the formation. The measured back-scatter flux is, in turn, primarily a function of the hydrogen content of the formation. If it is assumed that most hydrogen within the formation is contained in water or hydrocarbon in the pore space, the detectors respond to the porosity of the formation. As with the compensated density tool, the two neutron detectors respond to events at differing radial depths in the formation. The ratio of the detector response is formed to minimize the effects of reactions within the borehole, and porosity is determined from this ratio. The radial depth of investigation is about 9 or 10 inches, and the system can be calibrated to operate in both open and cased boreholes.
Cased and/or cemented borehole density logging can presently be done only with the borehole gravity technique. Logging with a gravimetry tool is time consuming and since it responds to a very large spatial volume of formation material, it has very poor depth resolution ("Well Logging for Physical Properties", J. R. Hearst and P. Nelson, Chapters 6 and 8, McGraw Hill, New York, 1985).
The pore space in formations is fully water saturated in formations below the water table, except when natural gas reservoirs are encountered. In natural gas bearing formations, a fraction of the pore space is partially liquid-saturated with some fraction containing water or possibly oil, and the remainder of the pore space is saturated with natural gas. Sometimes, the pressure of contained natural gas can be quite high. Above the water table, in the so called "vadose" zone, the pore space is also partially liquid saturated typically with water, with the remaining pore space containing air at near atmospheric pressure.
Prior art techniques for determining gas saturation of formations penetrated by an open borehole involve the combination of the responses of the conventional gamma-gamma type density tool and porosity sensitive neutron-neutron tool. When the density and porosity tools are calibrated for the water-saturated pore space condition, and when they log formations that are water-saturated, they will produce values for formation bulk density and formation porosity that are consistent with each other, assuming the tools are logging in a rock matrix that is the same as that used for calibration. However, when a formation zone is encountered where the pore water is replaced by gas, the porosity tool gives an erroneously low porosity indication, while the density tool correctly indicates a decrease in bulk density with corresponding apparent increase in porosity. This results in a "cross-over" of the log response curves from the two tools thereby indicating the presence of gas within the logged formation. This method is problematic in cased boreholes because of the more shallow investigation depth of the density log and its resulting greater sensitivity to variations in borehole conditions, such as variations in the thickness of the cement sheath, immediately behind the casing.
Gas has been detected in cased and cemented boreholes, with limited success, using a combination of a cased hole version of the compensated neutron-neutron logging tool and the gamma-gamma density tool. Cigni and Magrassi ("Gas Detection from Formation Density and Compensated Neutron Log in Cased Hole", M. Cigni and M. Magrassi, SPWLA 28th Annu. Logg. Symp., paper W, 1987) discuss the cased hole application of the neutron-neutron and gamma-gamma density logging tool combination. This method has been applied to the detection of gas migration in production and monitoring wells at the Prudhoe Bay fields of Alaska. Depth of investigation of the gamma-gamma measurement presents a problem as previously discussed. The neutron log also introduces a problem. Because the present day, commercial neutron-neutron porosity tool responds to changes in the thermal neutron distribution in the formation, which in turn is a function of hydrogen density within the formation, it is not able to distinguish between low porosity water-saturated formations and higher porosity partially gas-saturated formations since the hydrogen density in both formations can be the same. This is a serious disadvantage for the problem of formation gas detection, and is overcome by the density/gas saturation logging system set forth in this disclosure.
Logging for gas in cased, cemented boreholes is also performed using a logging tool containing a pulsed source of fast neutrons ("Examples of Dual Spacing Thermal Neutron Decay Time Logs in Texas Coast Oil and Gas Reservoirs", Trans. SPWLA 15th Annu. Logging Symp., 1979, and "The Use and Validation of Pulsed Neutron Surveys in Current Drilling Tests" Trans. SPWLA 19th Annu. Logg. Symp., 1978). This "pulsed-neutron decay time" or "pulsed neutron" tool, as it is known in the art, was designed to detect the presence of hydrocarbon liquids (oil) in formations where the water that otherwise fills the pore spaces is normally saline. This sensitivity to fluid type is achieved by measuring the formation thermal neutron cross section. However, the cross section is not very sensitive to the presence of gas, and therefore the logging tool is not very useful as a gas indicator. Another type of measurement can be performed with the pulsed-neutron decay time tool that is more sensitive to the presence of gas. This involves measuring a ratio of the tool's typically two axially spaced gamma detector responses. This ratio can, in turn, be interpreted in a manner that is sensitive to the presence of gas within the logged formation. Since the measured gamma radiation is produced by thermal neutron capture reactions, this response is similar to that of the neutron-neutron porosity log in that both are responding to changes in the spatial distribution of thermal neutrons which, in turn, is a function of hydrogen density. For this reason, the gamma ratio response, like the neutron-neutron porosity tool response, is not capable of distinguishing between low porosity formations and formations with higher porosity whose pore spaces are gas filled. The density/gas saturation logging system set forth in this disclosure, on the other hand, responds to the change in atom density and hence can distinguish between the gas saturated high porosity and liquid saturated low porosity formations.
The "carbon/oxygen" logging technique is well known in the art, and is used to delineate oil from fresh or saline water. Recently, the so called carbon/oxygen signal, which is "prompt" gamma radiation detected during the time of neutron production from the pulsed neutron type of logging tool, has been utilized to infer formation porosity ("Applications and Derivation of a New Cased Hole Density Porosity in Shaly Sand", R. C. Odum et al, Paper SPE 38699, annual Technical Conference and Exhibition, 1997). This tool has traditionally been used to detect the presence of oil in formations where the formation water has low salt content, and the pulsed neutron decay time log cannot distinguish oil from "fresh" water. More specifically, the system reported by Odum uses measures of inelastic scatter gamma radiation to determine water-saturated porosity of the logged formation. Porosity measurements using the carbon/oxygen logging equipment have been limited to the measurement of prompt gamma radiation, and do not involve the simultaneous measure of fast neutrons during the neutron burst.
U.S. Pat. No. 5,608,215 to Michael Evans discloses apparatus and methods for determining formation density by measuring and analyzing gamma radiation resulting from the irradiation of the formation with high energy neutrons. No method for simultaneously determining formation gas content is disclosed.
U.S. Pat. No. 5,539,225 to William A. Loomis et al discloses a measurement-while-drilling apparatus and method for determining formation density and gas. A high energy neutron source is used to irradiate the formation. Epithermal neutrons and gamma radiation are detected. The system is not suitable for use in cased boreholes.
U.S. Pat. No. 4,122,340 to Harry D. Smith, Jr. et al discloses a method for logging in which an earth formation penetrated by a borehole is irradiated with a pulsed source of fast neutrons, and the formation porosity is determined by making a dual spaced fast to epithermal neutron measurement. Prompt gamma radiation is not measured, and methods for distinguishing between high porosity gas filled zones and low porosity water filled zones are not disclosed.
U.S. Pat. No. 4,122,339 to Harry D. Smith, Jr. et al discloses a logging system which irradiates earth formations penetrated by a borehole with fast neutrons. Fast neutron population is inferred from inelastic scatter gamma radiation measured during bursts of fast neutrons from a pulsed neutron source using a gamma ray detector. An epithermal neutron detector is used to measure the epithermal neutron population following each neutron burst. Formation porosity is determined from the ratio of gamma ray detector to epithermal neutron detector responses. Two types of detectors (gamma ray and epithermal neutron) are used, and methods for distinguishing between high porosity gas filled zones and low porosity water filled zones are not disclosed.
U.S. Pat. No. 4,1134,011 to Harry D. Smith, Jr. et al discloses a method for logging in which earth formation penetrated by a borehole is irradiated with fast neutrons, and the formation porosity is determined by making a dual spaced fast to epithermal neutron measurement using a continuous source of fast neutrons. Prompt gamma radiation is not measured, and methods for distinguishing between high porosity gas filled zones and low porosity water filled zones are not disclosed.
U.S. Pat. No. 4,605,854 to Harry D. Smith, Jr. discloses a method for logging in which earth formation penetrated by a borehole is irradiated with fast neutrons, and the formation porosity is determined from a ratio of neutrons measured above 1 million electron volt (MeV) to the neutrons measured below 1 MeV. Gamma radiation is not measured, and methods for distinguishing between high porosity gas filled zones and low porosity water filled zones are not disclosed.
In view of the previously discussed prior art, an object of the present invention is to provide a logging system that measures the fractional gas saturation, fractional water saturation, and bulk density of earth material comprising the near-surface vadose zone surrounding boreholes that are open, that are cased, or that are cased and cemented.
Another object of the present invention is to provide a logging system that measures the bulk density and the presence of natural gas in earth material below the water table where the pore space is otherwise fully water saturated.
Yet another object of the present invention is to provide a logging system which measures water (or gas) saturation and formation bulk density in formations penetrated by cased and cemented boreholes in which conventional gamma-gamma and neutron-neutron log combinations yield erroneous results.
Still another object of the present invention is to provide a system which can delineate high porosity, gas saturated formations from low porosity, water saturated formations under all borehole conditions.
Another object of the present invention is to provide water (or gas) and bulk density measurements which "see" more deeply into the formation than conventional gamma-gamma and neutron-neutron logs.
Still another object of the present invention is to provide a single logging tool for performing all measurements required to obtain water (or gas) and formation bulk density measurements, thereby yielding better log interpretation and producing more accurate results than logs made with separate tools with differing investigation depths, as is the case with combining gamma-gamma and neutron-neutron logs.
Yet another object of the present invention is to provide a gas detection borehole log that has the potential for greater sensitivity to the presence of gas in the formation pore space than any prior art logging tools or combinations of responses of prior art logging tools.
Another object of the present invention is to provide a logging system which is safe to operate. The system uses a neutron generator source which can be turned off when not in use, as opposed to conventional gamma-gamma and neutron-neutron systems which use isotopic sources and therefore continuously emit gamma and neutron radiation, respectively. The present invention will be more easily licensed, and more acceptable to the user community for this reason.
A further object of the invention is to provide the porosity of earth material as computed from the measured bulk density and water saturation, given a knowledge of the material grain density.
Yet another object of the present invention is to provide a logging system which can simultaneously perform conventional logging measurements, such as thermal decay-time and neutron-neutron logs, for correlation with logs run previously in existing wells.
There are objects and applications of the present invention that will become apparent in the following disclosure.